There are numerous battery packs on the market that will do that. I've got this one which charges my HTC One V between 4 and 5 times from flat. Go for a higher capacity if you don't think that will be enough.
If you have a car with you, most phones now charge from a USB socket which are readily available to fit your car 12v socket. You can even recharge the battery packs mention above from the USB output. A combination of your car 12v and the battery pack will work really well.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets